[runtimes] Remove TOOLCHAIN_TOOLS specialization

{builtin,runtime}_register_target passes a TOOLCHAIN_TOOLS list, whereas
{builtin,runtime}_default_target does notl. The explicit TOOLCHAIN_TOOLS
list matches what LLVMExternalProjectUtils would have set anyway,
barring some target-specific adjustments, and those target-specific
adjustments seem valuable, so let's drop the explicit TOOLCHAIN_TOOLS
list and let LLVMExternalProjectUtils take care of it.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D86366